build_url($account, [ $transaction, [ $location, ] ] $fname, $lname);

##</code><code>##

sub build_url {
   my $lname = pop;
   my $fname = pop;
   my ($account, $transaction, $location) = @_;
   ...
}

##</code><code>##

sub build_url {
   my $account     = shift;
   my $transaction = @_>=4 ? shift : undef;
   my $locaton     = @_>=3 ? shift : undef;
   my $fname       = shift;
   my $lname       = shift;
   ...
}

##</code><code>##

sub build_url {
   splice(@_, 1, 0, (undef)x(5-@_))
   my ($account ,$transaction, $location, $fname, $lname) = @_;
   ...
}